One type of processor for treating workpieces with an RF plasma in a vacuum chamber includes a coil responsive to an RF source. The coil responds to the RF source to produce magnetic and electric fields that excite gas in the chamber into a plasma. Usually, the coil is planar and is on or adjacent to an exterior face of a dielectric window that extends in a direction generally parallel to a planar horizontally extending surface of the processed workpiece. Alternatively and/or additionally, the coil has a dome shape and the window has a dome shape or is planar. The excited plasma interacts with the workpiece in the chamber to etch, modify or deposit material on the workpiece, i.e., to process the workpiece. The workpiece is typically a semiconductor wafer having a planar circular surface or a solid dielectric plate, e.g., a rectangular glass substrate used in flat panel displays, or a metal plate.
Ogle, U.S. Pat. No. 4,948,458, discloses a multi-turn spiral coil for achieving the above results. The spiral, which is generally of the Archimedes type, extends radially and circumferentially between its interior and exterior terminals connected to the RF source via an impedance matching network. Coils of this general type produce oscillating RF fields having both magnetic and electric field components that penetrate through the dielectric window to heat electrons in the gas in a portion of the plasma in the chamber close to the window. The oscillating RF fields induce in the plasma currents that heat electrons in the plasma. The spatial distribution of the magnetic field in the plasma portion close to the window is a function of the sum of individual magnetic field components produced by each turn of the coil and by other currents, including those induced in the plasma. The magnetic field component produced by each of the turns is a function of the magnitude of RF current in each turn which differs for different turns because of dissipation, impedance and transmission line effects along the coil at the frequency of the RF source.
For spiral designs as disclosed by and based on the Ogle '458 patent, the RF currents in the spiral coil are distributed to produce a torroidal shaped magnetic field region in the portion of the plasma close to the window, which is where power is absorbed by the gas to excite the gas to a plasma. At low pressures, in the 1.0 to 10 mTorr range, global diffusion of the plasma from the ring-shaped region produces a substantially uniform plasma density just above the workpiece At intermediate pressure ranges, in the 10 to 100 mTorr range, the plasma density has a tendency to peak above the center of the workpiece which is coaxial with the coil. At high pressure, above 100 mTorr, gas phase collisions of electrons, ions and neutrons in the plasma reduce diffusion of the plasma charged particles outside the torroidal production region. As a result, there is a relatively high plasma flux in a torroidal region that is coaxial with and above the workpiece but lower plasma fluxes above the center and peripheral workpiece portions.
These differing operating conditions result in substantially large plasma flux (i.e., plasma density) variations between the torroid and the volumes interior and exterior to it, resulting in a substantially non-uniform spatial distribution of the plasma flux incident on the workpiece. The substantially non-uniform spatial distribution of the plasma flux incident on the workpiece has a tendency to cause non-uniform workpiece processing, i.e., different portions of the workpiece are etched to different extents and/or have different amounts of molecules deposited on them.
Many coils have been designed to improve the uniformity of the plasma produced by the Ogle type coil, see, e.g., the following United States Patents: Hama, U.S. Pat. No. 5,525,159; Okumura et al., U.S. Pat. No. 5,558,722; Barnes et al., U.S. Pat. No. 5,589,737; Okumura et al., U.S. Pat. No. 5,711,850; Hama et al., U.S. Pat. No. 5,716,451; Gates, U.S. Pat. No. 5,731,565; Holland et al., U.S. Pat. No. 5,759,280; Qian et al., U.S. Pat. No. 5,919,382; Holland et al., U.S. Pat. No. 5,800,619; Gates, U.S. Pat. No. 5,874,704; Holland et al., U.S. Pat. No. 5,975,013; Holland et al., U.S. Pat. No. 6,027,603; Khater et al., U.S. Pat. No. 6,028,285; Gates, U.S. Pat. No. 6,184,488; Holland et al., U.S. Pat. No. 6,268,700; Ni et al., U.S. Pat. No. 6,229,264; Qian et al., U.S. Pat. No. 6,297,468; Chen et al., U.S. Pat. No. 6,164,241; and Holland et al., U.S. Pat. No. 6,028,395. In many of these prior art patents, the coil includes plural windings connected in parallel between a pair of excitation terminals of the coil. The plural windings in some of the patents are interleaved, substantially co-planar spirals extending radially and circumferentially between the first and second excitation terminals respectively at the innermost and outermost portions of the coil.
Despite this extensive work, improved results are still possible in attaining uniform plasma density on a workpiece in a coil excited vacuum plasma processing chamber. None of these patents consider isolating the fields originating in one portion of the coil from: (1) the fields originating in other portions of the coil, (2) a region within the innermost turn of each of the windings, or (3) a region outside the outermost turn of each of the windings. The configurations in most of these coils having plural windings connected in parallel between a pair of coil excitation terminals are such that the plasma density in the center of the workpiece is substantially higher than the plasma density at portions of the workpiece beyond the workpiece center. While Holland et al. in U.S. Pat. Nos. 5,800,619 and 5,975,013 discloses a metal disk in the center of a coil including plural interlaced (i.e., interleaved), substantially coplanar parallel spiral windings, the purpose of the Holland et al. metal disk is to isolate from the remainder of the coil the fields associated with current flowing in the leads that are connected between a matching network and the coil interior and exterior terminals.
Okumura et al., U.S. Pat. Nos. 5,558,722 and 5,711,850, disclose helical coils including plural helical discharge elements arranged circumferentially at equal intervals, such that both ends of the of the helical windings are connected to first and second annular coils that are respectively connected to a high frequency power source and ground. The '722 and '850 patents also disclose multiple spiral type coils including spiral discharge coil elements connected to an annular coil and a normal spiral coil connected outwardly from the annular coil. Apparently, a common terminal at the interior of the spiral discharge coil elements is connected to one output terminal of a matching network and the end of the normal spiral coil is connected to ground. In another configuration of the '722 patent there is a multiple spiral type coil having inner ends connected to an annular coil. A normal spiral coil is connected inwardly from the annular coil. Apparently, the end of the normal spiral coil at the center of the configuration is connected to one output terminal of a matching network and the ends of the multiple spiral type coils at the periphery of the configuration are grounded. The '722 patent does not indicate that the annular coils thereof perform any shielding or field isolation function. Indeed, there are substantial magnetic fields outside and within the helical coil configurations, as well as outside of and in the center of the spiral coil configurations of the '722 patent.
It is, accordingly, an object of the present invention to provide a new and improved coil for a vacuum plasma processor.
Another object of the invention is to provide for a vacuum plasma processor a new and improved coil having multiple windings connected in parallel between a pair of coil excitation terminals, wherein the coil is arranged so that RF fields originating in one part of the coil are substantially decoupled from other parts the coil and/or from regions within the innermost and/or beyond the outermost portions of the coil.
An additional object of the invention is to provide for a vacuum plasma processor a new and improved coil having multiple windings connected in parallel between a pair of coil excitation terminals, wherein turns of the windings are arranged so that magnetic fields coupled from different portions of the windings have different magnetic flux densities to assist in providing a workpiece processing plasma having a relatively uniform density.
A further object of the invention is to provide for a vacuum plasma processor a new and improved coil having multiple windings connected in parallel between a pair of coil excitation terminals, wherein circuit elements connected to the windings cause different portions of the windings to couple magnetic fields having different magnetic flux densities to the plasma to assist in providing a workpiece processing plasma having a relatively uniform density.